1. Field of the Invention
The present invention relates to a writing controlling apparatus for controlling a current to be applied to a light source used in writing on an image carrier, an image forming apparatus including the writing controlling apparatus, and a computer program product including a computer-readable recording medium having a computer-readable program for executing a writing control.
2. Description of the Related Art
A laser printer performs writing by irradiating a photosensitive element that is an image carrier with a laser beam, thereby forming a latent image on the surface of the photosensitive element. The laser printer then performs developing by attaching toner on the surface of the photosensitive element on which the latent image is formed, and performs printing by transferring and fixing the image thus developed onto a paper sheet.
Because semiconductor lasers, which are inexpensive and small, are generally used in writing, laser printers includes a laser diode (LD), which is a semiconductor laser, and a driving circuit for driving the LD.
Available as schemes for driving an LD are a zero-bias modulation and a biased modulation. The zero-bias modulation is a scheme in which the LD is driven by a current corresponding to an input signal, while the bias current used for establishing a desired operating point is set to zero. By contrast, the biased modulation is a scheme in which the LD is driven by adding a current corresponding to an input signal to a bias current that is smaller than a threshold current, while the bias current is kept applied to the LD.
The biased modulation is currently often used because the biased modulation incurs a short delay time in generating a carrier at a density capable of oscillating a laser, so that it is possible to reduce the time required for the LD to start emitting light after a current corresponding to an input signal is started being applied.
Characteristics of an LD will now be explained briefly with reference to FIG. 1. FIG. 1 is a schematic diagram illustrating a relation between an input current and an optical output (light amount). The LD starts emitting light as the current is gradually increased. The light amount then increases moderately in proportional to the current level. The optical output then sharply increases when the current exceeds a certain level, and causes a laser oscillation. As a result, a laser beam is output from the LD. The current at the level at which the optical output sharply increases is referred to as an oscillation starting current.
A rate of this optical output change with respect to the current having exceeded the oscillation starting current is referred to as differential quantum efficiency, and is almost constant. When a line having the rate of this optical output change as a gradient is extended, as illustrated in a dotted line, the line intersects with a line of a zero optical output. The current level at this intersection serves as a threshold current, and is the lowest current capable of causing a laser oscillation. An appropriate bias current is then determined as a current equal to or lower than the threshold current. As an example, the bias current is determined as a threshold current×k, where k is a value equal to or more than 0.7 and equal to or less than 1.0.
The characteristics of an LD include the threshold current and the differential quantum efficiency, and these characteristics are known to vary greatly depending on the temperature. These characteristics are also known to vary depending on the time of a continuous LD operation.
Usually, immediately before printing is started, the light amount is measured while changing the current level, and the relation between the current and the light amount is acquired to allow these characteristics to be detected. The characteristics thus detected are used in determining an appropriate bias current level. The bias current level thus determined is set as a setting value, and the bias current to be supplied to the LD is controlled to the level specified as the setting value.
When the number of paper sheets to be printed is only one, a certain level of image quality can be ensured merely by detecting the characteristics of the LD immediately before the printing is started. However, when a plurality of paper sheets are to be printed successively, because the temperature of the LD increases and the time of a continuous operation is extended, the characteristics of the LD might differ greatly from those detected immediately before the printing is started, and the certain level of image quality can no longer be ensured.
In response to this problem, Japanese Patent Application Laid-open No. 2000-294871, for example, discloses a technology in which a temperature change and a time of a continuous operation are detected, and, if there is any change of a predetermined level, the detection of these characteristics (characteristics detection) is conducted again.
However, this technology requires some means for detecting a temperature change and a time of a continuous operation, and another characteristic detection is not conducted unless any change of a predetermined level is detected. Furthermore, a downtime (time in which printing is not executed) occurs while the characteristics detection is conducted again, disadvantageously.
Therefore, Japanese Patent Application Laid-open No. 2007-118521, for example, discloses a technology that allows characteristics detection to be conducted even when no change of a predetermined level is detected, without requiring such means, and also enabling a downtime to be avoided while the characteristics detection is conducted.
When a semiconductor laser performs scanning for a line in the main-scanning direction, the scanning starts at a point outside one end of the photosensitive element, travels across the photosensitive element, and reaches a point outside the other end of the photosensitive element, in the manner described later. The period during which the laser scans across the photosensitive element, within the period in which the semiconductor laser scans for one line, is referred to as an effective scanning period. The periods in which the laser scans the section outside of the one end and the section outside of the other end of the photosensitive element (the periods outside of the period during which the laser scans across the photosensitive element) are to be non-effective scanning periods.
In the technology disclosed in Japanese Patent Application Laid-open No. 2007-118521, in an interval between a paper sheet and another paper sheet each requiring a semiconductor laser to scan a plurality of lines, the semiconductor laser is caused to perform scanning in the effective scanning period and the non-effective scanning periods in order to conduct characteristics detection. A downtime is avoided by reflecting the result of the detection during the interval between these paper sheets.
However, in this technology, in order to enable characteristics detection, the photosensitive element during the interval between the paper sheets is irradiated with a laser beam in the effective scanning period, thereby forming a latent image on the photosensitive element. If the latent image is developed and transferred onto a paper sheet, horizontal lines are produced on the paper sheet.
If the latent image is transferred onto a section between the paper sheets without being transferred onto a paper sheet, the latent image is transferred onto a transfer belt carrying these paper sheets. The latent image is then transferred onto the rear side of a paper sheet fed onto the transfer belt, and causes the rear side to be smeared with toner.
Therefore, it has been desirable to provide an apparatus and the like that can avoid formation of horizontal lines and smearing of a rear side without incurring any downtime.